The invention relates to a device for the optical processing of electrical signals, and notably to a device that can be applied as a transversal filter or as a correlator of microwave signals.
More particularly, the invention relates to a set of fiber-optic devices that enable the processing of very wideband microwave signals and that carry out notably matched filter and correlator functions. These devices use the chromatic dispersion properties of optical fibers as well as the possibility of permanently inducing Bragg gratings therein.
It is known in the prior art that a tranvserse filter carries out a summation of samples of a signal, taken at different instants, with a weighting relationship characteristic of the signal to be filtered. Such a filter is used to determine, for example, the date of appearance of a signal p(t) of which there is a priori knowledge. This transient signal p(t) with a finite duration T is mixed with a noise b(t) that is independent of p(t). It is therefore the signal x(t)=p(t)+b(t) that has to be filtered. If such a filter maximizes the signal-to-noise ratio at the instant T, it is said to be matched. In the event of an ideal white noise, the pulse response h(t) of the matched filter is h(t)=p(-t): when the noise is not white, this filter is no longer optimal but makes it possible, however, to determine the date of appearance in most cases.
The weighting method described in the document by J. Max, Methodes et techniques du traitement du signal et applications aux mesures physiques ("Signal Processing Methods And Techniques And Applications To Physical Measurement"), Masson, 1987 is an exemplary embodiment of such a filter. As shown in FIG. 1, the signal x(t) feeds a delay line constituted by N elements, each giving a delay T. Furthermore, there is a sampling on N+1 points of the signal p(t): p(0), p(.tau.), . . . p(N.tau.). The signal coming from each element constituting the delay line is weighted by a coefficient .lambda..sub.k such that: EQU .lambda..sub.K =P((N-k).tau.)/.vertline.Pmax.vertline.
where .vertline.P.sub.max .vertline. is the maximum value of the modulus of p(t). At the instant t.sub.0, the sum y(t.sub.0) of the N+1 weighted outputs is equal to: ##EQU1##
This is really the result of the matched filtering at the instant t.sub.0 -T. This function is presently carried out by means of digital electronic devices, but is limited in frequency and cannot be used for the direct processing of signals at frequencies of the order of 20 GHz. Other approaches, which are analog approaches in this case, based on microwave guides or optical fibers such as those described in K. P. Jackson and J. J. Schaw, "Fiber-Optic Delay Line Signal Processors" in J. L. Horner ed. "Optical Signal Processing", Academic Press, make it possible to envisage attaining this frequency range, but they come up against the difficulty of making a large number of coupling points.
The invention relates to a device that can be used to obtain a large number of samples on very high frequency signals, typically n.apprxeq.1024 from 0 to 20 GHz.
Furthermore, it is often necessary in signal processing to compute the correlation product: ##EQU2## where R(t-t.sub.0) is an appropriately delayed reference signal,
S(t) is the signal to be correlated, PA1 T is the integration time, PA1 b is the noise equivalent power per Hz.
The object of this computation is to determine the value of t.sub.0 that ensures the maximum of the correlation function C(t.sub.0). It is thus necessary to have available a large number of samples of the reference signal delayed by different values of t.sub.0 in order to enable the precise determining of the value of t.sub.0 that maximizes C(t.sub.0). A function such as this can be obtained electronically, but it is limited to signals for which the frequency and the passband do not exceed some 100 MHz. This limitation is due to excessively slow sampling and to excessively low memory capacities.
Devices based on optical fibers, carrying out the correlation of two optically conveyed signals, have already been proposed (see for example, the French patent applications Nos. 87 10120 and 91 12040).